The present invention is directed in general to thin film multilayer antireflection coatings. It is directed in particular to a four-layer antireflection coating deposited in an in-line sputter coating apparatus by DC reactive sputtering.
In-line sputter coating apparatuses generally comprise a vacuum chamber including one or more sputtering cathodes. Two or more chambers arranged in-line may be connected together, for example to increase space available for sputtering cathodes. The sputtering cathodes may be planar magnetron cathodes, rotating cylindrical magnetron cathodes or a combination thereof. Each cathode includes a sputtering target of the material to be sputtered. A titanium target material, for example, may be reactively sputtered in an oxygen atmosphere to deposit a titanium dioxide (TiO.sub.2) layer. Cathodes having different target materials may be included in the apparatus for depositing coatings including layers of different materials. Such coatings are generally referred to as multilayer coatings.
A multilayer coating is deposited in an in-line system by introducing a substrate to be coated into the apparatus through a vacuum lock in one end of the chamber. The substrate is then transported past the cathodes, in turn, to deposit the coating, and then removed from a vacuum lock at the opposite end of the apparatus. The time required to coat a substrate by in-line sputtering is essentially only the time required to deposit the coating layer or layers. For an apparatus having a given number of cathodes, the speed with which a substrate may be coated and the cost of the coating is determined in large part by the rate at which material is sputtered from the cathode and deposited on the substrate.
In-line sputtering apparatus may be used to deposit high quality optical coatings such as antireflection coatings. Such coatings have formerly been deposited by thermal evaporation of coating materials in a vacuum. An apparatus used to deposit coatings by thermal evaporation is primarily a batch coating apparatus having only a single coating chamber. In batch coating, the substrates to be coated are loaded into a coating chamber, the chamber is evacuated, the substrates are coated, the chamber is vented, and the coated substrates are removed. Often substrates must also be heated in the vacuum chamber before they are coated, and coated substrates must be cooled before the chamber is vented for unloading the substrates. A coating cycle time, from loading to unloading substrates, may be two or more hours. Depositing the layers of the coating, however, may require only a few minutes.
The prior art teaching on coating designs, particularly multilayer antireflection coating designs, does not consider deposition speed in selecting coating materials. Materials are most often selected on the basis of advantageous optical or mechanical properties. For this reason conventional antireflection coating designs developed for thermal evaporation are not ideally suited for DC reactive sputter deposition.
Accordingly, it is an object of the present invention to provide an antireflection coating design optimized for deposition by DC reactive sputtering.
It is a further object of the present invention to provide an antireflection coating suitable for deposition in in-line sputtering apparatus.